Design strategy for the SLS accelerator facility

1. Quality
   
   A measure of the quality of a photon beam is its brightness, i.e. the number of photons per six-dimensional phase space volume (photons/s/0.1% BW/mm²/mrad²).
   High brightness has the following advantages:
   
   
   • short measuring time for a given experiment
     
     
   • measurement on very small samples (e.g. crystals of 30 µm)
     
     
   • microscopy and spectroscopy with excellent resolutions
     
     
   • high coherence of the synchrotron light allowing images with phase contrast
     
     
   • compact optical components (mirrors, monochromators), small aberration effects
   
   To get the desired high brightness the emittance of the circulating electron beam should be as low as possible. This means that the circumference of the storage ring should be, for a given energy, as large as possible and fully packed with bending magnets, quadrupoles and sextupoles. Given the restrictions in budget and the available area a circumference of 288 m was chosen for SLS.
   
   
2. Flexibility
   
   To serve a variety of users it is desirable to have a wide wavelength spectrum. To get hard X-rays the energy of the storage ring was chosen as 2.4 GeV, with the possibility to go as high as 2.7 GeV.
   For the corresponding undulator a straight section of 4 m is sufficiently long. On the other hand a long straight section of more than 10 m helps to reach low photon energies (ca. 10 eV) with an undulator.
   
   The SLS lattice was thus conceived with a mix of different insertion sections, namely:
   6 straights à 4 m,   3 straights à 7 m,   3 straights à 11.7 m
   
   To have full control of the beam size at every undulator and to optimize the so-called dynamic aperture, all 174 quadrupoles and 120 sextupoles are equipped with independent digital power supplies.
   
   
3. Stability
   
   
   • The primary step for stable photon beams was made already in the layout of the building: the concrete floor, housing the accelerator tunnel and the beamlines, was constructed as one large annular piece, and is mechanically separated from the rest of the building.
     
     
   • special care was taken in having very stable temperature conditions in the critical areas like the accelerator tunnel and the beamline hutches.
     
     
   • all multipole magnets of the storage ring are mounted and prealigned with great precision on steel girders.
     
     
   • An elaborate diagnostic equipment makes sure that the beam position is kept constant to within 1µm with a fast feedback system.
     
     
   • top-up injection from the booster into the storage ring keeps the intensity of the circulating electron beam approximately constant. The synchrotron light gives thus a constant heat load on the optical components such as mirrors, detectors etc. enhancing dramatically the stability of the photon beams used in the experimental areas.
     
     
   • In the accelerator system we use a novel type of high-precision digitally controlled power supplies.

List of Components